1. Field of the Invention
The present invention relates to a self light emitting display panel of a passive matrix drive method and a drive control method therefor in which for example organic EL (electroluminescent) elements are employed as light emitting elements and in which a display panel is divided into two display areas to perform dual scan display.
2. Description of the Related Art
Due to spread of cellular phones, personal digital assistants (PDAS), and the like, demand for a display panel which has a high definition image display function and which can realize a thin shape and low power consumption is increasing, and conventionally a liquid crystal display panel has been adopted in many products as a display panel which satisfies its needs. Meanwhile, these days an organic EL element which makes the best use of a characteristic being a self light emitting type display element has been employed for a manufactured product, and this has attracted attention as a next generation display panel instead of the conventional liquid display panel. This is because of backgrounds one of which is that by employing, in a light emitting layer of the element, an organic compound which enables an excellent light emission characteristic to be expected, a high efficiency and a long life which can be equal to practical use have been advanced.
The organic EL element is constructed basically in such a way that a transparent electrode for example by ITO, an organic EL medium, and a metallic electrode are laminated one by one on a transparent substrate such as glass or the like. The organic EL medium may be a single layer of an organic light emitting layer, a medium of double layer structure composed of an organic positive hole transport layer and an organic light emitting layer, a medium of a triple layer structure composed of an organic positive hole transport layer, an organic light emitting layer, and an organic electron transport layer, or a medium of a multilayer structure in which an injection layer of electron or positive hole is inserted into an appropriate portion among these layers.
The above-described organic EL element can be electrically replaced by a structure composed of a light emitting component having a diode characteristic and a parasitic capacitance component which is connected in parallel-to-this light emitting component, and thus the organic EL element can be said to be a capacitive light emitting element. When a light emission drive voltage is applied to this organic EL element, at first, electrical charges corresponding to the electric capacity of this element flow into the electrode as a displacement current and are accumulated. It can be considered that when the drive voltage then exceeds a determined voltage (light emission threshold voltage=Vth) peculiar to this element, current begins to flow from one electrode (anode electrode side of the diode component) to an organic layer constituting the light emitting layer so that the element emits light at an intensity proportional to this current.
Regarding the organic EL element, due to reasons that the voltage-intensity characteristic thereof is unstable with respect to temperature changes while the current-intensity characteristic thereof is stable with respect to temperature changes and that degradation of the organic EL element is considerable when the organic EL element receives an excess current so that light emission lifetime is shortened, a constant current drive is performed in general. As display panels in which such organic EL elements are employed, a passive drive type display panel in which the elements are arranged in a matrix pattern has already been put into practical use in some products.
FIG. 1 shows a basic structure of a conventional passive matrix type display panel and a drive circuit therefor. Regarding drive methods for organic EL elements in this passive matrix drive method, there are two methods of cathode line scan/anode line drive and anode line scan/cathode line drive, and the structure shown in FIG. 1 shows a form of the former cathode line scan/anode line drive. That is, anode lines A1–Am as m data lines are arranged in a vertical direction, cathode lines K1–Kn as n scan selection lines are arranged in a horizontal direction, and organic EL elements E11–Emn designated by symbols/marks of diodes are arranged at portions at which the anode lines intersect the cathode lines (in total, m×n portions) to construct a display panel 1.
In the respective EL elements E11–Emn constituting pixels, one ends thereof (anode terminals in the equivalent diodes of the EL elements) are connected to the anode lines and the other ends thereof (cathode terminals in the equivalent diodes of the EL elements) are connected to the cathode lines, corresponding to the respective intersection positions between the anode lines A1–Am extending along the vertical direction and the cathode lines K1–Kn extending along the horizontal direction. Further, one end portions of the respective anode lines A1–Am are connected to a data driver 2, and one end portions of the respective cathode lines K1–Kn are connected to a scan driver 3, so as to be driven respectively.
The scan driver 3 allows the cathode lines K1–Kn connected thereto to connect for example to a reference potential point (ground) sequentially alternatively, and the data driver 2 operates to allow pixels by the EL elements to emit light selectively by appropriately supplying light emission drive current to the respective anode lines A1–Am in synchronization with the scan selection.
Meanwhile, in a display panel by this type of passive matrix drive method, as a panel size is increased, a line resistance or line capacitance increases, and thus a RC response time increases. Since a signal delay due to the increase of the RC response time not only deteriorates response (response operation) of image display in a display but also delays the time until the voltage reaches a light emission threshold voltage during the scan time in respective light emission elements, it causes a substantial light emission intensity of the display to be decreased. In order to solve such a problem, dual scan in which for example a display panel is divided into two sections, the upper and lower, and in which respective display panels are scanned simultaneously, that is, a dual scan method, has been proposed.
In a case where the dual scan method is adopted, since scanning operations for two divided display panels can be respectively implemented simultaneously, the scan time for each scan line can be set to a longer period of time, and a light emission time rate (light emission duty) of a light emitting element can be increased. Therefore, even when drive current given to a light emitting element is decreased to decrease momentary light emission intensity of the element, the brightness of a display screen can be ensured satisfactorily. The dual scan drive method is disclosed in Japanese Patent Application Laid-Open No. 2003-302937 shown below.
FIG. 2 shows examples of operations of cases where the dual scan drive method is adopted, and as this dual scan drive method, scan control methods shown in FIG. 2A or FIG. 2(B) have been considered. In FIG. 2A or FIG. 2(B), n scan lines (n is a natural number of a multiple of 2) arranged on a panel are divided into an upper half and a lower half to constitute display panels 1A and 1B.
In FIG. 2, as scan lines respectively counted from the top are represented by numbers on right sides of the respective display panels 1A and 1B, the upper half of the panel 1A has scan lines from a first scan line to a (n/2)th scan line, and these are driven to emit light by unillustrated data driver and scan driver corresponding to the upper half of the panel 1A. The lower half of the panel 1B has scan lines from a (n/2+1)th scan line to an nth scan line, and these are driven to emit light by unillustrated similar data driver and scan driver (not shown) corresponding to the lower half of the panel 1B.
Here, in the scan control method shown in FIG. 2A, the first through (n/2)th scan lines of the upper half are scanned from the first line toward the (n/2)th line sequentially, and at the same time the (n/2+1)th through nth scan lines of the lower half are scanned from the (n/2+1)th line toward the nth line sequentially. That is, the arrows shown in a left side in FIG. 2A show a scan direction for scanning the respective panels of the upper half and the lower half.
The scan control method shown in FIG. 2(B) shows an example for scanning the respective panels in a reverse direction with respect to the direction described above. That is, in the scan control method shown in FIG. 2(B), the first through (n/2)th scan lines of the upper half are scanned from the (n/2)th line toward the first line sequentially, and at the same time the (n/2+1)th through nth scan lines of the lower half are scanned from the nth line toward the (n/2+1)th line sequentially. That is, the arrows shown in a left side in FIG. 2(B) show a scan direction for scanning the respective panels of the upper half and the lower half.
Meanwhile, even when any of the scan control methods shown in FIGS. 2(A) and 2(B) is adopted, in a case where a figure laid across the upper half and the lower half is displayed to move fast for example in a horizontal direction, trouble as described below occurs. FIG. 3 shows an example of a case where the respective panels 1A, 1B of the upper half and the lower half are simultaneously scanned in a downward direction from the top as shown in FIG. 2A. FIG. 3A shows a state in which a blocky figure F displayed to be laid across the upper half and the lower half is being displayed on the right side of the screen, and FIG. 3B shows a condition that in a next frame the blocky figure F is moved to a central portion of the screen to be displayed as shown by the outlined arrow.
FIG. 4 schematically explains movements of lit pixels with respect to the respective scan lines resulting from a moving representation of the blocky figure F as shown in FIG. 3 and is schematic views of the lit pixels regarding which the vicinity of the boundary laid across the upper half and the lower half is enlarged and shown. FIGS. 4A to 4G show the movements of the lit pixels during one frame period. Since a display panel employing self light emitting elements represented by the above-mentioned organic EL elements for pixels has a so-called normally black characteristic, although normally a non-illuminating state is shown by black and an illuminating state is shown by white, the relationship of the black and white is reversed and shown in FIG. 4 for convenience of illustration.
FIG. 4A shows a state in which the blocky figure F is displayed on the right side of the screen as shown in FIG. 3A. In the state shown in this FIG. 4A, upon the start of scanning, since scanning of the first line in the lower half of the panel, that is, the (n/2+1)th line, is first implemented, pixels on the (n/2+1)th line are moved to a central portion of the screen to be lit as shown by the outlined arrow in FIG. 4B. Next, since scanning of the (n/2+2)th line is implemented, pixels on the (n/2+2)th line are moved to a central portion of the screen to be lit as shown by the outlined arrow in FIG. 4C.
Further, similarly, since scanning of the (n/2+3)th line is implemented in the next step, pixels on the (n/2+3)th line are moved to a central portion of the screen to be lit as shown by the outlined arrow in FIG. 4D. During the period of FIGS. 4A to 4D described above, since the first through third lines are scanned sequentially from the top in the upper half of the panel, lit pixels in the vicinity of the boundary in the upper half of the screen do not move to a central portion of the screen.
At the time of the state shown in FIG. 4E in the vicinity of the end of one frame as scanning for each line progresses, since scanning the (n/2−2)th line in the upper half is implemented, here, for the first time, lit pixels in the upper half are moved to a central portion of the screen to be lit as shown by the outlined arrow. Following that, since scanning the (n/2−1)th line in the upper half of the panel is implemented, pixels on the (n/2−1)th line are moved to a central portion of the screen to be lit as shown by the outlined arrow in FIG. 4F.
Further, since scanning the (n/2)th line is implemented at the end of one frame period, pixels on the (n/2)th line are moved to a central portion of the screen to be lit as shown by the outlined arrow in FIG. 4G. Thus, as shown in FIG. 3B, the blocky figure F is moved to a central portion of the screen to be displayed.
As is apparent from the description above, a period from the completion of the movement of the lit pixels displayed on the lower half of the panel as shown in FIG. 4D to the start of the movement of the lit pixels displayed on the upper half of the panel as shown in FIG. 4E requires a period close to one frame. Since this is recognized as an after image in human vision, the figure is recognized with a sense of incompatibility that the figure is divided into two although the figure is one blocky figure. Although a relatively simple operation of a case where a blocky figure is moved from a right end to a central portion of a screen is exemplified in the description above, in reality complex figure changes such as a movement further to a left side of the screen or rapid reciprocating movements may occur. In such a case, the above-described sense of incompatibility may be perceived further considerably.
Thus, in order to prevent the above-described sense of incompatibility from occurring, as shown in FIG. 5, it can be considered to adopt a means for scanning the first through (n/2)th scan lines from the first line to the (n/2)th line sequentially (that is, from the upper end to the center of the screen) in the upper half of the panel and at the same time for scanning the (n/2+1)th through nth scan lines from the nth line to the (n/2+1)th line sequentially (that is, from the lower end to the center of the screen) in the lower half of the panel. The arrows displayed on a left side in FIG. 5A show scan directions for scanning the respective upper half and lower half of the panel.
FIGS. 5A to 5D show states in which during a period of one frame as shown in FIG. 3 the blocky figure F which is displayed so as to be laid across the upper half-and the lower half is moved from the right side to the central portion of the screen to be displayed similarly to the example already described. That is, FIGS. 5A to 5D show movements of lit pixels during the period of one frame.
According to the scan method shown in FIG. 5, until a time just before the completion of one frame period, as shown in FIG. 5A, there is no movement of lit pixels in the upper half and lower half of the panel. Immediately before the completion of scanning, as shown in FIG. 5B, since the (n/2−2)th line in the upper half of the panel and the (n/2+3)th line in the lower half of the panel are simultaneously scanned, pixels of the lines corresponding to these are respectively moved to the central portions of the screen to be lit.
At the next scan timing, as shown in FIG. 5C, pixels of the (n/2−1)th line in the upper half of the panel and the (n/2+2)th line in the lower half of the panel are simultaneously moved to the central portions to be lit. Similarly, at a scan timing of the end of one frame, as shown in FIG. 5D, pixels of the (n/2)th line in the upper half of the panel and the (n/2+1)th line in the lower half of the panel are simultaneously moved to the central portions to be lit.
Although there occurs a state in which one blocky figure F is divided in a time domain so that divided ones move on the screen even when the scan method shown in FIG. 5 is adopted, a time period required for the movement of the entire block becomes an extremely short time compared to the case where the scan method shown in FIG. 4 already described is adopted. Accordingly, an afterimage effect in human vision is hard to occur, and a problem that a sense of incompatibility occurs as in the example shown in FIG. 4 can be resolved.
Meanwhile, in the case where the scan method shown in FIG. 5 is adopted, the lowermost scan line (the (n/2)th scan line) in the upper half of the panel and the uppermost scan line (the (n/2+1)th scan line) in the lower half of the panel are brought to a scan selection state simultaneously. That is, the pixels on the vertically adjoining scan lines emit light. In such a case, there occurs a problem that the two scan lines are recognized as a line brighter than normal in human vision.
Japanese Patent Application Laid-Open No. 2003-302937 shown earlier as a prior art reference describes that momentarily intense light is emitted in the case where scanning is implemented in the same direction in the upper half and the lower half of the panel, that is, in the case where the scan method shown in FIG. 2A or 2(B) already described is adopted. However, the ground thereof is not made clear. Yet, the occurrence of bright line corresponds to the case as shown in FIG. 5 in which the scan method in which a central portion is treated as an axis of symmetry is adopted, and the present inventors have confirmed in experiments that in the case where adjacent scan lines are in the scan selection state, light emission is recognized further extensively in human vision.